GPS has been widely adopted in the fields of navigation and positioning. Through GPS, extremely accurate three-dimensional position, velocity, and timing information can be attained by users equipped with GPS receivers anywhere in the world. Basically, a constellation of orbiting GPS satellites continuously broadcasts encoded signals back to earth. Each of the twenty-four GPS satellites transmits its encoded signal containing relevant identification, position, and time information. A ground-based or airborne GPS receiver is then used to pick up and process the encoded signals received from a number of these GPS satellites. Based on the received GPS information, the receiver can obtain a position fix with an accuracy of approximately ten meters in each axis.
Although a ten-meter level of accuracy is exceptional in light of the relative distances involved, there are many applications, such as collision avoidance, surveying, mapping, tracking, etc., which require a much higher degree of accuracy. One technique for improving the accuracy is known as "differential" GPS. In differential GPS, a base station or beacon is placed at a known position (e.g., as determined by traditional surveying techniques). A GPS receiver housed within the base station measures its position based on GPS signals received from the satellites. Since the true position of the base station is already known, the differences between the measured versus the actual position can easily be determined. This difference represents the aggregate errors attributable to various natural, uncontrollable sources (e.g., satellite clock error, ephemeris error, tropospheric and ionospheric delay error, noise/quantization error. Once these errors have been determined, their effects can be minimized by applying realtime corrections. The base station continuously calculates and broadcasts updated realtime corrections. Assuming that a nearby remote "roving" GPS receiver also experiences similar errors, it can compensate for these errors by adjusting its measurements according to the realtime correction information received from a nearby base station or beacon. With standard real time differential GPS, position accuracies of 1-5 meters can be attained. A more detailed description of differential GPS is given by Blackwell, Overview of Differential GPS Methods, Navigation: Journal of The Institute of Navigation, Vol. 32, No. 2, Summer 1985, pp 114-125 and U.S. Pat. Nos. 5,523,763 and 5,495,257.
Unfortunately, there are several limitations associated with differential GPS. One main limitation is the fact that the base stations are required to have a data transmission link with a speed of data transmission at a certain baud rate set forth in a defined specification. By today's standards, the specified baud rate appears to be quite low; however, it is well-established and universally adopted. This specified baud rate is insufficient and does not have the capacity to transmit the amount of differential correction information associated with the multitude of satellites necessary for maximum error compensation. Furthermore, it takes time for the base station to process and update the differential correction information, resulting in transmission delays. In addition, other errors, such as interpolation, ionospheric and tropospheric propagation errors, base station receiver errors, synchronization and other timing errors, may exist.
In order to minimize these errors, a technique known as "postprocessed" GPS was developed. With conventional postprocessed GPS, the measurement data gathered in realtime in the field by a roving GPS receiver is brought back to a central office and downloaded to a computer system. The computer system has access to a large database containing comprehensive GPS information relating to a number of base stations. Specially designed software compiles all of this information and applies rigorous processing to the downloaded data gathered in realtime. After performing iterative processing, postprocessed corrected position fixes are determined. In this manner, many of the latencies and timing errors associated with differential GPS are minimized, resulting in accuracies in the range of approximately less than 1 meter.
Although conventional post-processed GPS is extremely accurate, it suffers from several drawbacks. First, full measurement data are typically logged. This requires a high-capacity disk drive for storing the large amount of data or other forms of media are used--Ram, Flash memory, etc. Otherwise, the user would be required to make many trips back and forth to the field. Another disadvantage is that since full post-processing requires iterative recomputation of complete position/velocity fixes, a lengthy amount of time is expended and expensive, sophisticated computing resources are required.
In light of these and other problems, there is a need for a fast, efficient, and yet highly accurate method for determining post-processed positions/velocities. The present invention provides one solution, whereby the accuracies of differential GPS positions/velocities are improved with minimal amount of additional processing. The present invention also optimizes data transmission while minimizing the storage requirements of position/velocity, time, and differential correction information. Furthermore, the present invention actually provides a method to adjust realtime differential GPS positions and velocities with computed (postprocessed) differential measurement corrections without the need for logging full measurement data, and also without the need of iterative recomputation of complete position/velocity fixes during conventional postprocessing.